Table of Contents

• Testing differences
• 1: How to convince your boss
• 2: Looking at the data
• 3: Is the average different?
• 4: How big is big?
• 5: How to combine SDs
• 6: Combining SDs for fun and profit
• t-calc and t-crit
• 7: Getting to t-calc
• 8: A formula for t-calc
• 9: Degrees of freedom
• 10: The bottom line (finally)
• Putting it all together
• 11: Give it a try
• 12: Once more for luck
• 13: The big picture

How big is big ? If you said standard deviation...

you've either got a good memory or you're definitely telepathic.

Do you remember how to calculate a standard deviation (SD)? If not, there's a quick review below.

Of course, the easy way to do this is to use your graphing calculator, assuming that's ok with your instructor. However, it is important to understand what the standard deviation is doing. Basically, it is a slightly-dressed-up version of how far each datapoint is from the group average. The first fish gained 212g, or 55 grams less than the average. The last fish gained 324g, or 57 grams more than the average. These are the "deviations" from average.

Unfortuantely, we can't just average the deviation. Instead, we square them (partly to get rid of the plus or minus signs), average them (except divide by n-1 rather than n), and square root that number. And that's the standard deviation.